1. Field of the Invention
The present invention relates to a controller of an electric power steering device for assisting steering operation of a vehicle operator by an electric motor connected to a steering system and, more specifically, relates to improvement of steering control performance while a vehicle is running on a road surface with a low friction coefficient by correcting a reaction force torque based on a reaction force of a steering system in an electric power steering device that gives a reaction force torque according to a steering angle of the steering system.
2. Description of the Related Art
Conventionally, there is known an electric power steering device in which an electric motor is connected to a steering system for converting a steering angle applied to a steering wheel by a vehicle operator into a steering angle of road wheels so that steering force of a vehicle operator may be reduced by adding a driving force of this electric motor to the steering system. FIG. 24 illustrates a typical example of such an electric power steering device.
The electric power steering device shown in FIG. 24 is provided with a pinion 4 that is coupled to a steering shaft 2, which is integrally bound to a steering wheel 1, via a connecting shaft 3 having universal joints 3a and 3b, a manual steering force generating mechanism 9 consisting of a rack and pinion mechanism composed of a rack shaft 8 which can mesh with the pinion 4 to move reciprocatingly in the vehicle width direction and both ends of which are coupled to knuckle arms 7, 7 (in FIG. 24, only one of two is shown) of right and left front wheels 6, 6 via tie rods 5, 5, an electric motor 10 that is arranged coaxially with and connected to the rack shaft 8 to provide an assisting steering force for reducing a steering force generated by this manual steering force generating mechanism 9, a steering force detecting means 11 for acting on the pinion 4 and detecting a manual steering force of a vehicle operator, a steering angle detecting means 12 for detecting a rotating angle of the steering wheel 1 and controlling means 13 for controlling an output of the electric motor 10 based on a detection value TP of the steering force detecting means 11 and a detection value θ of the steering angle detecting means 12.
As illustrated in FIG. 25, the controlling means 13 is provided with an output target value generating means 14 for setting a target torque value that should be produced by the electric motor 10 and outputting the target torque value and electric motor driving means 15 for driving and controlling the electric motor 10 based on the target torque value outputted by this output target value generating means 14. The controlling means 13 thus controls an assisting steering force which the controlling means 13 causes the electric motor 10 to generate based on the output TP of the steering force detecting means 11.
Incidentally, in the above-mentioned electric power steering device of the conventional type, in case of steering on a road surface with a low friction coefficient such as a road with accumulated snow, there is a problem in that the assisting steering force tends to be excessive due to a small reaction force of the road surface, which causes a sense of incongruity to the vehicle operator.
Japanese Patent Application Laid-open No. Sho 64-74168 discloses a power steering device which is configured such that a rotational resistance can be given to a steering wheel under predetermined running conditions as a device so as to improve such an inconvenience, that is, the tendency for the vehicle operator to excessively cut the steering wheel 1 on a slippery road.
However, the resistance to the rotation of the steering wheel also resists the rotation of the steering wheel as it returns to its neutral position, and this could cause a substantial discomfort to the vehicle operator. However, since this technique controls the rotational resistance which is added to the steering system according to, for example, a road surface friction coefficient, the resistance is also added to the operation of returning the steering wheel to its neutral position.
Moreover, Japanese Patent Application Laid-open No. Hei 9-58506 discloses an electric power steering control device that controls an output of an electric motor for generating power, which is added to a manual steering system, based on a steering force value to be added to the manual steering system by a vehicle operator and a maximum permissible steering angle value of the manual steering system which is set based on a friction coefficient of a road surface.
However, since this technique controls an output torque of the electric motor to reduce an assisting force by the electric motor to increase a steering reaction force as the steering angle of the manual steering system approaches the limit steering angle (maximum permissible steering angle value), the vehicle operator experiences difficulty in getting a feeling of a slippery road surface and, further, tends to excessively cut the steering wheel on the slippery road surface. Moreover, although it is necessary to detect a friction coefficient of a road surface, left and right road wheels often have different friction coefficients of a road surface, for example, in a case where snow remains only on shoulders of the road. In addition, it is difficult to accurately detect a friction coefficient of a road surface because it changes every moment.